The term “fancy” refers to diamonds that are yellow or brown in color more intense than "Z" or diamonds that display any other shade outside of yellow or brown. The system for grading fancy colored diamonds is similar to colored gemstones, such as sapphire, ruby and emerald, and break down the overall hue, saturation, and tone of color
For example, a light Purple diamond will not be designated with a color grade of "H", "M", or "T", it will receive a color grading of Faint Purple, Very Light Purple or Light Purple.
Color Scale for Diamonds in the "D"-"Z" range:
The spectrum of color is condensed into a list of 27 predominant color hues that gem grading laboratories, such as GIA, use to describe colored gems and diamonds:
The value for fancy diamonds is based upon different factors than those used for regular diamonds. White diamonds are graded on a scale from an icy white “D” through a light yellow “Z”, where, as you move closer to “D”, the diamond becomes rarer and more valuable. On the other hand, in fancy colored diamonds, the more intense and vivid the color is, the rarer and more valuable it becomes.
Celebrity bling and fashion trends also affect the demand and value of certain diamond colors.
For instance, purple diamonds increased in popularity after Kobe Bryant presented a rare 8 carat purple diamond to his then wife, Vanessa, following a scandal in 2003. Ben Affleck proposed to Jennifer Lopez with a 6.1 carat pink diamond engagement ring from Harry Winston in 2002 and suddenly many girls were lusting for a pink diamond of their own. Creative marketing of Dark Brown diamonds from LeVian, has created a demand for this formerly less desired shade by calling them "Chocolate Diamonds".